Many transmission standards are known for transmitting signals over cables. Some of these standards allow signals to be transmitted over very large distances, for example such as those employed in telephony. Many of these transmission standards involve the modulation of signals, so as to present a modulated signal to the transmission medium which can be transmitted over much longer distances through said medium.
Over shorter lengths, for example, within a building or data processing facility, it is desirable to transmit signals in an unmodulated form, thereby avoiding the need for a multiplicity of modulating and demodulating devices. However, in this form, signal degradation is more likely to occur, as the lengths of cable increase until a critical point is reached at which the cable is too long for the signal to be transmitted.
In such an environment, a common problem for technicians and engineers is that of determining the extent to which a cable may be extended before critical degradation does occur. A known technique for making this assessment consists of inserting lengths of cable into the circuit and assessing whether the equipment continues to function correctly with said additional length in place. A major problem with this approach is that even a relatively short additional length of cable can be quite heavy and bulky and to perform a suitable test, it may be necessary to connect several different lengths of cable. Thus, the problem is not essentially one of determining how the test may be performed but of physically implementing the test, given that cables may be less than perfectly accessible and the space available for making such tests may be limited.
An environment where such a problem often occurs is in a television studio or video editing facility, in which digital video signals and, more recently, serial digital video signals, are transmitted between various processing devices, such as editing desks, recording equipment and effects machines etc.
In accordance with the CCIR 601 standard, 10 bit parallel data may be converted to a serial bit stream consisting of 270 megabits per second transmitted by scrambling the information in accordance with an non-return to zero algorithm. Alternatively, composite video in NTSC or PAL format, may be transmitted in accordance with the D2 standard at 144 megabits per second or 177 megabits per second respectively.
As previously stated, the transmission of such signals over physical cables will result in signal attenuation and the level of attenuation will depend upon the quality of the cable and its length. However, the level of attenuation and signal delay also varies with the transmission frequency of the signal and, being digital in nature, the transmitted waveform will be made up of a plurality of different frequency components. Thus, these frequency components will tend to be attenuated and delayed by differing amounts, resulting in signal distortion.
Being a digital signal, the degree of attenuation and distortion introduced by the cable will not result in appreciable signal degradation until a certain level has been attained, whereafter, the degradation becomes unacceptable and the resulting noise introduced to the signal results in an unacceptable number of errors being introduced.
Thus, in assessing the extent to which the length of the cable may be increased, it is necessary to keep adding different lengths of cable and then determine whether an acceptable or an unacceptable level of degradation is occurs.